The present invention relates to integrated circuits and to methods for manufacturing them.
Self-aligned refractory metal-silicide technology has been recognized as one of the keys to realizing good device performance in integrated circuits as device dimension scales down. Titanium disilicide (TiSi.sub.2) has become recognized as one of the most attractive metal-silicides, because of its low resistivity, stability, and capability for self-aligned formation.
One of the major advantages of titanium silicide technology is the availability of a self-aligned VLSI process. That is, by depositing a layer of titanium metal overall and then heating in a nitrogen atmosphere, all exposed areas of silicon (whether monocrystalline or polycrystalline) will react to form titanium silicides, and a composition dominated by titanium nitrides will be formed where the titanium metal was not in contact with silicon. This is tremendously useful, since, by performing this step after the polysilicon gate level has been patterned, silicide will be formed on the surface of exposed source/drain regions (or other exposed substrate surface regions), on the surface of the polysilicon gate level, and nowhere else. This means that the source/drain diffusions can be made shallower while still preserving an acceptably low sheet resistance, and also means that the sheet resistance of the polysilicon gate level can be lowered. The use of the nitrogen atmosphere in this process is critical, since otherwise silicon will outdiffuse through the growing silicide layer and permit lateral growth, so that the titanium silicide formed by this reaction will be able to bridge gaps of a half micron or so, e.g. between gate and source/drain of a VLSI device. Such a process is described in, for example, M. Alperin et al., Development of the Self-aligned TiSi.sub.2 Process for VLSI Applications, at page 141 of the February 1985 issue of the IEEE Transactions on Electron Devices, which is hereby incorporated by reference.
However, this self-aligned TiSi.sub.2 technology is degraded by any oxygen contamination of the nitrogen atmosphere used for the silicide react process. It has been found that oxygen contamination during the react process can result in two problems: first, the silicide would have high resistivity; second, the unreacted titanium (or non-silicide material such as titanium nitride) was hard to strip off.
In general, titanium dioxide has some chemical properties which are quite inconvenient in integrated circuit fabrication. TiO.sub.2 has an extremely high heat of formation, and is accordingly an extremely stable compound. Etching of TiO.sub.2 is very difficult, and any oxygen present during the nitridation step is very likely to form TiO.sub.2, which is more energetically favorable than TiN. Since TiO.sub.2 is an insulator, unlike TiN or TiSi.sub.2, TiO.sub.2 formation can cause drastic degradation in electrical properties, and is not easily reversed.
In order to avoid possible oxygen contamination, the manufacturing process will normally either use a very high flow rate of high purity purging gas such as argon (along with the high purity N.sub.2 supply) or use low pressure in the react environment. Both of these methods increase the production cost and, in practice, are hard to maintain. The use of Rapid Thermal Processing to form TiSi.sub.2 does alleviate some of the contamination concerns, but gas flow rates and purity still play an important role in achieving uniform silicidation.
The present invention minimizes the problems of oxygen contamination by placing a cap oxide on top of the titanium layer before the silicide react. During the react process, this oxide is partially reduced by the adjacent layer of titanium metal. Therefore, some oxygen will be freed and can diffuse into the titanium layer. In the low temperature stage of the react process, the oxygen is gettered at the grain boundaries of the titanium metal and will retard the silicon atom from outdiffusion across grain boundaries. The introduced oxygen impurity in this process apparently performs a function partially analogous to that of the nitrogen used in the traditional self-aligned TiSi.sub.2 process.
The reasons why oxygen derived from the oxide layer does not lead to the same problems as free oxygen contamination would (difficulty in stripping, and poor sheet resistance of the silicide) are not perfectly understood. However, it is suspected that the activation energy of forming TiO.sub.2 may play an important role. The redox reaction which forms titanium dioxide and silicon from titanium and silicon dioxide will have a much smaller enthalpy than the reaction of titanium with gaseous oxygen, and the activation energy of the redox reaction may also be higher. Moreover, the composition of the titanium dioxides is likely to be different; titanium reacting with free O.sub.2 will typically form rutile (an extremely stable form of TiO.sub.2), whereas oxygen diffusing along the grain boundaries of the titanium metal may form a much more weakly bound compound.
It should also be noted that titanium metal has an extremely high affinity for hydrogen, so it is possible that hydrogen already present at the grain boundaries of the titanium may cooperate in some fashion with the oxygen diffusing down from the cap oxide to limit silicon diffusion.
In any case, it has been experimentally demonstrated that the titanium/oxygen composite under the cap oxide will block silicon outdiffusion during the siliciding step, can be converted to a composition predominantly comprising titanium nitride during a nitrogen atmosphere anneal, and can be converted to titanium silicide if heated while in contact with silicon. Thus it seems clear that the oxygen in this location is in some form which is less stable than rutile TiO.sub.2.
An alternative embodiment of the present invention teaches that the titanium layer should be capped with an oxide/nitride stack, to provide further protection against oxygen indiffusing through the cap oxide.
A key advantage of the titanium silicide process is that local interconnects are readily available. That is, as described in grandparent application Ser. No. 729,318, filed 05/01/85, pending. During the nitrogen-atmosphere directreact process the portions of the titanium which are not in contact with silicon will form a composition which principally comprises titanium nitride (TiN), although this composition will typically not be perfectly stoichiometric. If local interconnects are not needed this TiN layer can simply be stripped: but if desired the TiN layer can be patterned and etched to provide very thin low-sheet-resistance local interconnects.
The parent application (Ser. No. 837,468, filed 03/07/86, pending) described, among other embodiments, one embodiment in which the TiN layer was patterned by patterning a cap oxide over the as-deposited titanium layer. The structure with the patterned cap oxide in place was then heated in nitrogen to form silicides, all exposed TiN was stripped, and then the cap oxide was stripped and the remaining titanium (which had been protected by the cap oxide) was again heated in nitrogen to form TiN local interconnects. That application described the surprising chemical interactions between the cap oxide and the titanium which permitted this process to work, and described the composition of the resulting local interconnects. The present invention is generally compatible with this process, except that the cap oxide layer (which defines the local interconnect pattern) is patterned and etched after the silicidation reaction rather than before it.
Another feature of the nitrogen atmosphere process is that competing reactions are occurring over silicon: titanium nitride grows downward from the gas phase as titanium silicide grows up from the silicon interface. Since these competing reactions have unequal activation energies, the TiN/TiSi.sub.2 thickness ratio is a sensitive function of temperature. Since the TiN will be removed except at local interconnect locations, Ti consumed by TiN formation lowers the sheet resistance of the silicide layer. This means that sheet resistance is a sensitive function of reaction temperature, which is undesirable.
Thus, the present invention provides at least the following advantages, in addition to others mentioned in this application:
1. Since the process requires no interaction between the titanium and ambient gas such as nitrogen, it can easily be made immune to oxygen contamination by making the cap material a nitride/oxide stack. Therefore, no special requirements need be imposed on the react ambient. This greatly reduces processing cost and improves the reproducibility of film quality.
2. Since there is no titanium nitride formed during the react, the sheet resistance of the TiSi.sub.2 layer is less dependent on the react temperature than in the nitrogen atmosphere process.
3. The capability for titanium nitride local interconnect is not affected by this method. Titanium nitride interconnects can easily be fabricated, by selectively etching away the unreacted titanium and then performing another thermal nitridation in a noncritical nitrogen ambient.
4. The final anneal of the TiSi.sub.2 could be performed in nitrogen ambient to produce a stacked titanium nitride/TiSi.sub.2 layer, where the titanium nitride could be used as a good aluminum diffusion barrier. This method will produce thicker titanium nitride than the nitrogen-atmosphere process without sacrificing the sheet resistance of the TiSi.sub.2, since no titanium material on the silicide layer is stripped away before the anneal.
According to the present invention there is provided: A method for fabricating integrated circuit devices, comprising the steps of: providing a substrate having thereon a partially fabricated integrated circuit structure including exposed portions consisting essentially of slicon; depositing overall a metal layer consisting predominantly of titanium; providing a dielectric layer directly overlying at least predetermined portions of said metal layer, said dielectric layer consisting predominantly of oxides; and applying heat until portions of said metal layer react with portions of said exposed silicon to form titanium silicides; whereby said dielctric layer suppresses silicon outdiffusion through portions of said metal layer under said dielectric layer.
According to the present invention there is also provided: A method for fabricating integrated circuit devices, comprising the steps of: providing a substrate having thereon a partially fabricated integrated circuit structure including exposed portions consisting essentially of silicon; depositing overall a metal layer consisting predominantly of titanium; providing a dielectric layer directly overlying at least predetermined portions of said metal layer, said dielectric layer consisting predominantly of oxides; and applying heat until portions of said metal layer react with portions of said exposed silicon to form titanium silicides; whereby said dielectric layer suppresses silicon outdiffusion through portions of said metal layer under said dielectric layer; removing predetermined portions of said dielectric layer and/or of the remainder of said metal layer in a predetermined pattern to define local interconnects; and applying heat in a nitrogen atmosphere to convert remaining portions of metal layer to a conductive material consisting predominantly of titanium nitride.
According to the present invention there is also provided: A method for fabricating integrated circuit devices, comprising the steps of: providing a substrate having thereon a partially fabricated integrated circuit structure including exposed portions consisting essentially of silicon; depositing overall a metal layer consisting predominantly of titanium; providing a dielectric layer directly overlying at least predetermined portions of said metal layer, said dielectric layer consisting predominantly of oxides; and applying heat until portions of said metal layer react with portions of said exposed silicon to form titanium silicides, whereby said dielectric layer suppresses silicon outdiffusion through portions of said metal layer under said dielectric layer; removing said dielectric layer; applying heat in a nitrogen atmosphere to convert remaining portions of metal layer to a conductive material consisting predominantly of titanium nitride; and etching said conductive material in a predetermined pattern to provide local interconnect lines.